public void Permutations()
        {
            var expected = new[]
            {
                new[] { 1, 2, 3 },
                new[] { 1, 2, 4 },
                new[] { 1, 2, 5 },
                new[] { 1, 3, 4 },
                new[] { 1, 3, 5 },
                new[] { 1, 4, 5 },
                new[] { 2, 3, 4 },
                new[] { 2, 3, 5 },
                new[] { 2, 4, 5 },
                new[] { 3, 4, 5 },
            };

            Assert.Equal(expected, MatrixExtensions.Permutations(Enumerable.Range(1, 5), 3));
        }